In a high bandwidth digital transmission system, such as in an advanced digital television (ADTV) system, it is important that artifacts introduced into the received signal by the transmission channel, such as multipath effects and intersymbol interference, be removed, or proper reception will be difficult or impossible. To remove such artifacts, ADTV receivers may include an adaptive equalizer. Such adaptive equalizers are generally embodied as multitap transverse digital filters in an FIR and/or IIR configuration with tap coefficients which are continually varied to minimize the transmission channel artifacts and to maintain the received signal as close as possible to the transmitted signal. In general, the larger the number of taps, the better the equalization quality.
Once the adaptive equalizer is in operation, various decision directed processes are known for maintaining the coefficients as close to their optimum values as practicable. However, these decision directed processes require that the coefficients already be relatively close to their optimum values in order to operate properly. If the values of the coefficients are too far from their optimum values, these decision directed processes fail to converge. Thus, proper operation of an adaptive equalizer requires two steps, an initial adjustment of the coefficients close to their optimum values, then maintenance of the coefficients as close to their optimum values as practicable.
In some high bandwidth systems, such as telephone systems, a training signal is transmitted before data transmission, and the adaptive equalizer can adjust the coefficients close to their optimum values based on the received training signal. The nature of a television system, however, is such that training signals may be impractical. Thus, the adaptive equalizer must generate the initial estimates of the coefficients based on the received data signal alone, a process known as blind equalization. Only when the blind equalization process has been completed can the decision directed adaptive equalization process be invoked to maintain the coefficients at or near their optimum values.